The present invention pertains to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission display screens.
Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This allows the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, the vacuum tube of the FED can be made of glass much thinner than that of conventional CRTs. Moreover, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pocket-TVs and portable electronic games.
As mentioned in FEDs and conventional CRT displays differ in the way the image is scanned. Conventional CRT displays generate images by scanning an electron beam across the phosphor screen in a raster pattern. As the electron beam scans along the row (horizontal) direction, its intensity is adjusted according to the desired brightness of each pixel of the row. After a row of pixel is scanned, the electron beam steps down and scans the next row with its intensity modulated according to the desired brightness of that row. In marked contrast, FEDs generate images according to a xe2x80x9cmatrixxe2x80x9d addressing scheme. Each electron beam of the FED is formed at the intersection of individual rows and columns of the display. Rows are updated sequentially. A single row electrode is activated alone with all the columns active, and the voltage applied to each column determines the strength of the electron beam formed at the intersection of that row and column. Then, the next row is subsequently activated and new brightness information is set again on each of the columns. When all the rows have been updated, a new frame is displayed.
Beside the difference in image scanning methodology, a more significant difference between FEDs and conventional CRT displays is that conventional CRT displays emit electrons with xe2x80x9chotxe2x80x9d cathodes, while FEDs utilize xe2x80x9ccoldxe2x80x9d cathodes. For instance, in a conventional CRT display, a metal composite is heated to about 1200 xc2x0 C. to emit electrons. These electrons are then focused into a tight beam and accelerated towards the phosphor screen. In contrast, FEDs generate a high electric field by applying a voltage across a very narrow gap between emitter-tips and emitter-gates to emit electrons. Because it is not necessary to expend thermal energy to emit electrons, xe2x80x9ccoldxe2x80x9d cathodes consume far less power than xe2x80x9chotxe2x80x9d cathodes.
One drawback of the xe2x80x9ccoldxe2x80x9d cathodes, however, is that emission efficiency of the electron emitters is moderately unstable. The electron emitters may degrade after several hours of continuous operation, resulting in a lower emission current and a dimmer display. Some electron emitters may degrade faster than others, resulting in a display having uneven luminance across the screen. Naturally, these visual artifacts are highly undesirable for a high-quality flat panel display.
Therefore, what is needed is a system for and method of extending the operational life of FEDs. What is further needed is a. system for and method of extending the operational life of FEDs that can be implemented without redesigning the entire FED screen and remain cost-effective.
The present invention provides for a field emission display having an improved operational life. In one embodiment of the present invention, the FED comprises a plurality of row lines, a plurality of column lines, and a plurality of electron emissive elements disposed at intersections of the plurality of row lines and column lines, a column driver circuit, and a row driver circuit. The column driver circuit is coupled to drive column voltage signals over the plurality of column lines; and, the row driver circuit is coupled to activate and deactivate the plurality of row lines with row voltage signals. Significantly, according to the present invention, operational life of the FED is substantially extended when the electron emissive elements are intermittently reverse-biased by the column voltage signals and the row voltage signals.
In one embodiment of the invention, electron emissive elements are coupled to the row lines and gate electrodes are coupled to the column lines. According to this embodiment, the row driver circuit is configured for providing a row-off voltage that is pre-set at a relatively more positive voltage than a column-off voltage to deactivate the row line. In this way, when a row line is deactivated and when the column lines are driven below the row-off voltage, electron emissive elements disposed between the row line and the column lines are reverse-biased. Alternatively, the xe2x80x9coffxe2x80x9d voltage may be set above a column full-on voltage such that electron emissive elements are reverse-biased whenever the row line is deactivated.
In another embodiment of the present invention, electron emissive elements are coupled to the column lines, and the gate electrodes are coupled to the row lines. In that embodiment, the row driver circuit is configured for providing a positive row-on voltage to activate a row line, and a row-off voltage that is relatively less positive than a column-off voltage provided by the column driver circuit to deactivate the row line. Reverse-biasing of the electron emissive elements is achieved when the row line is deactivated and when the column lines are driven above the row-off voltage. Alternatively, the row-off voltage may be set below a column full-on voltage to reverse-bias the electron emissive elements when the row line is deactivated.
In yet another embodiment of the present invention, the row driver circuit and the column driver circuit are responsive to a SLEEP signal. The column driver circuit, upon receiving the SLEEP signal, drives a first sleep-mode voltage over the column lines. The row driver circuit, upon receiving the SLEEP signal, drives a second sleep-mode voltage over the row lines. According to the present embodiment, the first and second sleep-mode voltages, when asserted, cause the electron emissive elements to be reverse-biased. According to one embodiment of the invention, in FEDs where the row lines are coupled to the electron emissive elements, the second sleep-mode voltage is more positive than the first sleep-mode voltage. In another embodiment, in FEDs where the column lines are coupled to the electron emissive elements, the second sleep-mode voltage is less positive than the first-sleep mode voltage.
In furtherance of one embodiment of the present invention, electronic circuitry of the FED further comprises a controller circuit for receiving the SLEEP signal. In this embodiment, the controller circuit is configured for providing a first set of reference voltages to the row driver when the SLEEP signal is not asserted, and for providing a second set of reference voltages to the row driver when the SLEEP signal is asserted. The row driver then drives the row lines with appropriate normal-mode and sleep-mode voltages in response to the different sets of reference voltages.
In accordance with another embodiment of the present invention, the FED may include circuit means for measuring an emission current, and circuit means for adjusting the voltage difference between the row-off voltage and the column-off voltage according to a difference between the emission current and a reference current. In this way, emission efficiency of the electron emissive elements may be maintained at a constant level via a feedback mechanism.
Embodiments of the present invention include the above and wherein the electron emissive elements further comprises conical electron emissive elements each having a molybdenum tip. In addition, the FED of the present invention may include opto-isolation circuits for converting external signals corresponding to the first set of reference voltages to signals corresponding to the second set of reference voltages to be provided to the row driver circuit.